(a) Field of the Invention
The present invention relates generally to intrasatellite communications. More particularly, it relates to a wireless method of intrasatellite communication that uses infrared transceivers to replace hardwired electrical connections.
(b) Description of Related Art
Conventional satellites are designed to collect, process, and transmit a multitude of digital and analog signals. Typically, hundreds of signals must be coordinated and distributed within the satellite. Conventional intrasatellite signal distribution systems are based on a hierarchical hardwired arrangement of data collection and transmission devices.
A conventional hierarchical satellite signal distribution system 10 is depicted in FIG. 1. Telemetry and command signals are collected from and distributed to a variety of transducers, sensors, and user units 11 that are located within various compartments or sections of the satellite. User units 11 are distinguished from individual (i.e. stand alone) sensors or transducers in that they comprise a more complex functional module within the satellite that typically requires and produces a plurality of telemetry and command signals. Telemetry signals may be bi-level (i.e. 1 bit digital), analog, conditioned analog, and digital words (multi-bit digital). Bi-level signals may, for example, represent the on/off operational status of a sub-system or functional module within the satellite. Analog signals may, for example, be a voltage value that represents the RF power level of a microwave signal in a communication channel, or may alternatively be the filament voltage of a traveling wave tube. Conditioned analog signals may, for example, be a voltage produced across a resistive temperature sensor in response to a precise current source. A multi-bit digital signal may, for example, represent the angular position of the satellite's antenna with respect to the earth.
Telemetry signals within a given compartment or section of the satellite are typically connected via wires to a remote command and telemetry unit (RCTU) 13. Each section or compartment of the satellite may contain one or more RCTUs 13. The RCTUs 13 aggregate and digitize various analog and digital telemetry signals received from the sensors, transducers, and user units 11, and forward them to a central command and telemetry unit (CCTU) 15 along a high speed data bus 14. The CCTU 15 further aggregates the telemetry data received from the RCTUs 13 and produces a multiplexed data stream to be modulated on an RF carrier and transmitted to earth. The CCTU 15 may also forward the multiplexed data stream to an on-board computer system for ground scheduled or other autonomous action by the satellite.
Thus, telemetry signals provide a way for monitoring current satellite conditions remotely from the earth and from on-board the satellite itself. Based on these telemetry signals, the satellite may take autonomous actions and issue command signals to its various user units. Alternatively, satellite actions may be remotely invoked from the earth. In either case, command signals are received by the CCTU, routed via the high-speed data bus to the appropriate RCTU, distributed via wires to the appropriate user unit, and finally processed by the appropriate user unit to execute the desired action. Commanded actions may, for example, include turning on a function, routing a signal to a new destination, or initiating an action in response to an external event.
A single satellite may contain hundreds of user units that each produces and receives a plurality of telemetry and command signals. As a result, in the conventional intrasatellite communications approach hundreds of signal interconnections made between the RCTUs, user units, and stand alone devices are hardwired.
Several disadvantages are inherent in the conventional hardwired approach described above. One disadvantage is that hardwired systems are susceptible to capacitively or inductively coupled electrical transients, and conducted electrical surges and noise. In particular, the increased use of lightweight materials such as Kapton, Teflon, and fused silica to accommodate launch vehicle constraints have increased susceptibility to electrostatic discharge (ESD) events. Satellite appendages such as solar arrays and large diameter antennas can produce ESD events in the kilo-amp range, which can easily damage or destroy sensitive electronics on-board the satellite. Careful design of the satellite's structure and the liberal use of shielding (e.g. Faraday cages) can reduce the potential for damage, but cannot substantially eliminate it.
Another disadvantage of the conventional hardwired approach to intrasatellite communication is that the low cost of wire is more than offset by the higher manufacturing and deployment costs that the mass of wires generates. Hardwiring significantly increases the labor costs associated with producing a satellite because hundreds of electrical/mechanical wire connections must be made by hand. Furthermore, these hand connections are likely to produce manufacturing defects that result in expensive troubleshooting and rework or that, more significantly, may produce a latent defect that manifests itself after the satellite has been placed into orbit. Additionally, factory testing and debugging of satellite sub-systems is made more difficult by hardwired connections because physical connections must be reliably and repeatedly made between each sub-system and various factory functional test systems. Finally, hardwired systems require a large mass of wires that can increase deployment costs because a more expensive launch vehicle may be required to carry the additional weight.